Particle detectors are devices to detect, track and/or identify radiation or particles, and find wide applications throughout particle physics, biology as well as medical technology.
Particle detectors exploiting the process of ionization and charge multiplication in gases have been in use with continued improvements ever since Rutherford first employed a gas-filled wire counter to study natural radioactivity in 1908. Methods for obtaining large stable proportional gains, increased resolution and greater robustness to sparks and discharges in gaseous detectors are a continuing subject of investigation in the detector community today.
Gaseous detectors typically collect the electrons released by ionizing radiation and guide them to a region with a large electric field, thereby initiating an electron avalanche. The avalanche is able to produce enough electrons to create a current or charge large enough to be collected on a readout electrode assembly and analyzed by readout electronics. The collected electron charge may indicate the charge, energy, momentum, direction of travel and other attributes of the incident particles.
In most such detectors, the large amplification field necessary to initiate and support the electron avalanche comes from a thin wire at a positive high voltage potential. This same thin wire also collects the electrons from the avalanche and guides them towards the readout electronics. More recently, attention has focused on so-called micropattern gas detectors (MPGDs) such as the Micromesh Gaseous Structure Chamber (MicroMegas) and the Gas Electron Multiplier (GEM). By employing semiconductor techniques, large area tracking MPGDs can be mass-produced in an impressive variety of geometries while at the same time allowing small avalanche gaps, and hence rapid signal development, fast readout and high reliability. In MPGDs, the electrons generated in the amplification process are typically collected on metallic readout pads or strips arranged in a predetermined pattern on a semiconductor substrate and electrically connected to fast readout electronics.
In order to protect the readout pads, the detector substrate and the readout electronics, as well as to better insulate neighboring pads, it has become conventional to cover the readout pads with a thin resistive layer. However, finding a material with the right resistivity properties and still allowing easy manufacturing proved to be difficult. Moreover, organic resistive layers that have conventionally been employed as cover layers are rather sensitive to sparks or discharges in the amplification gap, and hence prone to early aging when operated at high counting rates or high amplification fields. Mineral resistive layers are more robust to sparks and discharges, but require complex manufacturing, and are hence less suitable for mass-production of large area detectors.
Sparks may not only damage the readout detector surface, but also inevitably lead to dead times during which events cannot be detected, hence reducing detector efficiency.